Nervous system

Proper formation of synapses in the developing nervous system is critical to the expected function and behavior of an adult organism. Neurons must project neurites, in the form of axons or dendrites, to target areas to complete synaptic circuits. The biochemical tool that cells use to interact with the external environment and direct the guidance of developing neurites are guidance receptors. One such guidance receptor that is extensively studied to uncover its roles in developmental disorders and disease is DSCAM (Down-Syndrome Cell Adhesion Molecule). To better understand the role of DSCAM in humans, a fly homolog Dscam1 was extensively characterized in the giant fiber system (GFS) of Drosophila to further explore its roles in axon guidance, synapse formation, and synapse function. The UAS-Gal4 system was used to alter the protein levels of Dscam1 within the giant fiber interneurons (GFs). A UAS-RNAi construct against Dscam1 was used to knockdown translation of all possible isoforms within the GFs. A UAS-Dscam1(TM2) construct was used to overexpress a single isoform of Dscam1 that is specifically trafficked to the axons. Confocal microscopy was used to determine the morphological changes associated with dysregulated Dscam1 levels. Visualization via fluorescent markers was accomplished of both pre- and post-synaptic cells, the GFs and tergotrochanteral motorneurons (TTMns), respectively, and synapse interface was determined as colocalization of the two cells. Additionally, the functional components of the GF-TTMn synapse, both gap-junctions, and presynaptic chemical active zones were tagged via fluorescent antibodies and quantified.

Astrogliosis may contribute to the pathogenesis of CNS dysfunction as in Alzheimer's Disease or Sudden Infant Death Syndrome (SIDS). Investigating astrocytic expression in the brain may provide valuable insights into and/or models of CNS disease or injury. This study was designed to test astrocyte expression and distribution in the mature rat and immature rat and cat brain utilizing different fixatives (aldehyde or ethanol), immunocytochemistry (glial fibrillary acidic protein, GFAP, or vimentin), and lesion conditions in the cortex, cerebellum, and brainstem. Findings include a paucity of GFAP positive astrocytes in most regions of the normal rat brainstem, evidence of astrogliosis in lesions, and the most intense, reactive astrocytes in the 7 day post-lesion condition. The results imply that the rat (brainstem) may make a suitable animal model for investigating the role of hypoxia and astrogliosis in neural trauma such as postulated in SIDS.

Huntington's Disease (HD) is a devastating neurodegenerative disorder caused by an expanded polyglutamine repeat within the Huntingtin gene IT15. In this study we demonstrated that Bcl-2 interacting mediator of cell death Extra Long (BimEL) protein expression was significantly increased in cells expressing mutant Huntingtin (mHtt). Moreover, striatal BimEL expression remained high in an R6/2 HD mouse model throughout the disease progression. Utilizing novel BimEL phospho-mutants we demonstrated the phosphorylation of Ser65 to be important for the stabilization of BimEL. We provided evidence that impaired proteasome function, increased JNK activity and reduced striatal BDNF lead to changes in the phosphorylation of BimEL, thereby promoting its stabilization specifically within the striatum of R6/2 mice. Furthermore, knocking down BimEL expression prevented mHtt-induced cell death in a HD cell culture. Taken together, these findings suggest that BimEL may contribute to the selective neurodegeneration and pathogenesis of HD.

A method for modeling and simulating neural action potential (AP) propagation along the length of an axon containing a number of Ranvier nodes is proposed in this dissertation. A system identification approach is adopted to represent node of Ranvier (NR) response to current pulse stimulus in the form of transfer function representations for NR excitability. Segments of myelinated internodal (IN) and NR regions are cascaded, representing the remaining downstream axon after a site-of-stimulus introduction of an external current pulse. This cascading network is used to simulate "cable" properties and signal propagation along the length of the axon. This work proposes possible solutions to attenuation losses inherited in the classical myelinated cable models and accounts for neuronal AP velocity as well as introducing signal attenuation and transient delays associated with internodal demyelination. This model could aide as a predictive tool for the diagnosis and analysis of axonal signal integrity associated with demyelination pathology. Possible applications could include functional stimulation control methodologies for axon bundles that may exhibit signal fidelity issues associated with demyelination. It is further proposed that this model may serve as an instructive tool for further development and incorporation of other axon dynamic behaviors such as: relative refractory periods of AP generation, NR AP recovery mechanisms and responses to varied current stimulus input.

Alzheimer's disease (AD) is an increasingly common neurological disorder that mainly affects memory formation and retention. It is characterized by unique intercellular neurofibrillary tangles and extracellular beta-amyloid plaques. Histone deacetylase inhibitors (HDACi's) are competitive antagonists against histone deacetylases, causing histone acetyltransferases to acetylate the genome unregulated. This thesis investigates the use of new histone deacetylase inhibitors on recovering memory in a mouse model of Alzheimer's disease. By use of a fear conditioning paradigm, we have shown that these HDACI's increase memory in AD mice, but show either no effect or a positive effect in wild-type mice. Future experiments will investigate the efficacy of compound 966 and the spine density of hippocampal brain slices after fear conditioning trials.